It has recently become known that oils and fats, particularly those containing polyunsaturated fatty acids, have physiologic activity. Accordingly, such oils and fats have been increasingly and widely used in food and animal feed as additives, from the viewpoint of health. Eicosapentaenoic acid and docosahexaenoic acid are polyunsaturated fatty acids mainly contained in fish oil. It has been found that they have the effect of, for example, preventing hyperlipemia, high blood pressure, skin aging, and the like, and they have been used in medical drugs and food with health-promoting benefits. Unfortunately, oils and fats containing such a polyunsaturated fatty acid have low oxidative stability. Accordingly, food to which such an oil or fat can be added is limited, or if added to food or the like, the oil or fat undesirably generates an odor due to its oxidation, even slight oxidation. How the oil or fat is handled needs to be taken into account. For example, refined fish oil remaining after use must be hermetically sealed with the can of the fish oil filled with nitrogen gas. Thus, there are limits in use, such as of product type, distribution temperature, content, and storage conditions.
Since, for example, docosahexaenoic acid and eicosapentaenoic acid play an important role to develop the brain and retinas and the memory and learning function of babies and breast milk contains these fatty acids, modified milk for babies to which fish oil containing docosahexaenoic acid and eicosapentaenoic acid has been added is commercially available. Also, since arachidonic acid plays an important role for growth and is contained in breast milk, addition of arachidonic acid to the modified baby milk has been attempted. However, it is necessary to take care not to oxidize polyunsaturated fatty acids when these polyunsaturated fatty acids are blended into the modified baby milk. Otherwise, oxidized odor is generated by oxidation, so that not only the modified milk becomes difficult to ingest, but also the modified milk itself may be degraded to be toxic.
Encapsulated eicosapentaenoic acid ethyl esters are commercially available as medical drugs for oral administration. Refined fish oil containing eicosapentaenoic acid and docosahexaenoic acid is also available in form of capsule as health food. Since these fatty acids are liable to be oxidized, their use is limited, except for use in capsules.
In order enhance the oxidative stability, the oils and fats can be powdered. For example, oil or fat may be encapsulated into microcapsules to be powdered, or enclosed with cyclodextrin and powdered so that stable powdered oil or fat is provided. However, this approach makes the production steps complicated and decreases productivity. In addition, capsules may be broken during storage, and the type of capsule applicable for food and animal feed is limited, disadvantageously.
In order to enhance the oxidative stability of oils and fats, various types of antioxidant have been used. For example, plural types of antioxidant are used in combination, or a synergist, such as phosphoric acid, citric acid, or ascorbic acid, is added to an antioxidant to enhance the antioxidant properties. However, the oxidation stabilities of fish oils and other oils and fats having extremely low oxidative stability cannot be sufficiently enhanced by only such combinations of antioxidants and synergists.
Dibutylhydroxytoluene (BHT), butylhydroxyanisole (BHA), gallic acid, propyl gallate, tocopherols, and the like are approved as antioxidants for oils and fats and food containing oil or fat, in order to prevent oils and fats from oxidizing. In medical drugs and the like, synthetic antioxidants are used, such as BHT, BHA, TBHQ, and ethoxyquin.
Sesame oil is relatively stable to oxidation, and it has been known since a long time ago that sesame contains antioxidative components, such as sesamol and other lignans (Japanese Unexamined Patent Application Publication No. 58-132076; Shoku no Kagaku, 225 (11) pp. 40-48 (1996); Shoku no Kagaku, 225 (11) pp. 32-36 (1996)).
Sesamol is an antioxidant for oils and fats and food containing oil or fat, and is approved as a food additive. It is however reported that sesamol is not effective for oils and fats exhibiting extremely low oxidative stability, such as fish oil (NOF Corporation, from fiscal Heisei 4 (1994) to Heisei 8 (1996), DHA Koudo Seisei Chushutsu Gijutsu Kaihatsu Jigyo, Kekka Gaiyou (DHA Koudo Seisei Chushutsu Gijutsu Kenkyu Kumiai) pp. 74-79 (2002)). Sesamol is not used for enhancing the oxidative stability of fish oil.
Ascorbic acid and ascorbic acid derivatives are also approved as food additives and used as antioxidants for oils and fats and food containing oil or fat. However, they are not effective for oils and fats exhibiting extremely low oxidative stability, such as fish oil, if they are used alone, and even their combined use with tocopherol does not produce satisfactory effects.
In order to prevent the oxidation of oils and fats, combined use of various types of antioxidant has been attempted. For example, Japanese Unexamined Patent Application Publication No. 2002-142673 has disclosed a lipophilic antioxidant prepared by emulsifying gallic acid, a water-soluble antioxidant, and an oil-soluble antioxidant into a water-in-oil form with a lipophilic emulsifier. In this application, examples of the water-soluble antioxidant include vitamin C, citric acid, chlorogenic acid, their derivatives, sugar-amino reaction products, proanthocyanidin, flavone derivatives, tea extracts, grape seed extracts, and rutin, and examples of the oil-soluble antioxidant include tocopherol, ascorbyl palmitate, sesamol, and γ-oryzanol.
Effects of antioxidants have been compared for pyrolysis of tocopherol in vegetable oils in Nippon Eiyo Shokuryo Gakkaishi (Journal of Japanese Society of Nutrition and Food Science), 44 (6) pp. 493-498 (1991), 45 (3) pp. 291-295 (1992), and 45 (3) pp. 285-290 (1992). Although some of the antioxidants use sesamol and an ascorbic acid ester in combination, they do not produce effects particularly superior to other antioxidants. These literatures discuss effects in oxidation of vegetable oils (having 3 or less unsaturated bonds) at high temperatures, but not in oxidation of polyunsaturated fatty acids (having at least three unsaturated bonds) during storage at room temperature; hence different objects are used under different conditions. This is probably because the combination of sesamol and an ascorbic acid ester does not produce superior effects. In addition, the thermal instability of the antioxidants may affect antioxidant properties.
Demand for oil or fat containing an unsaturated fatty acid is increasingly growing. Accordingly, it is highly desired to solve the problem of oxidative stability, including how to handle the oil or fat, in a strict sense.